The present invention relates to a method for determining a track of a geographical trajectory.
The track which is obtainable by the method of the invention can for example be used for including the location or path of a whole trajectory in a global coordinate system, for determining the length of a trajectory, for obtaining information about specified sections of a trajectory, for determining the geographical location of a specified event or other purposes.
A method for determining a track is for example known from FR-A-2610100, which relates to a method and device for determining deformations and the path of a pipe. The method known from FR-A-2610100 comprises the steps of moving a data collecting vehicle, which comprises sensors such as accelerometers and gyroscopes, through the pipe, collecting and storing samples during the movement and afterwards transferring the collected samples to a computer for determining the length and horizontal projection profile of the pipe. A known problem of sensors such as accelerometers and gyroscopes is that for measurements over a long period of time, the measurements become inaccurate due to a drift of the sensors, i.e. a build-up of errors. There is no teaching in FR-A-2610100 of how such drifts may be compensated or, in other words, how the accuracy of the collected data can be improved.
EP-A-534338 and US-A-2002/0005297 relate to methods and devices for steering or navigating a boring machine. In these methods, a sensing device comprising sensors such as accelerometers and gyroscopes is attached to the head of the boring machine, which communicates in real time with a steering unit. The steering unit sends steering signals to the head in response of signals from the sensing device. Due to the drifts of the sensors, it is necessary to reset the sensing device at given time intervals to re-establish a proper heading or orientation of the head. In US-A-2002/0005297, this resetting is performed by detecting the location of the sensing device by means of a tracker unit. In EP-A-534338, the resetting is performed by detaching the sensing device from the head and moving it to a reference location with known coordinates. There is no teaching in EP-A-534338 nor US-A-2002/0005297 of how accuracy of the sensor signals can be improved without having to reset the sensing device.
GB-A-2351807 relates to a method for well bore surveying, which makes use of a probe with an inertial measurement unit. The wellbore is surveyed by retrieving the probe from the wellbore bottom and determining the incremental positions of the probe during the retrieval using the inertial measurement unit. Some error correction is applied by referring to coordinates of a known survey endpoint and referencing to the velocity of the wireline on which the probe is suspended, but there is no teaching to apply error correction to the measurement data without the aid of this external information.
GB-A-2331811 relates to a method for well bore surveying by means of rate gyro and gravity measurements. A sonde is moved incrementally through the borehole. At each location, the rate gyro of the sonde takes two orthogonal measurements by rotating the gyro 180°, which are averaged for removing an instrument drift. This error compensation is derived from comparing two measurements of the same physical quantity and is as such limited to a single measurement instrument. Furthermore, this method requires a standstill of the sonde for rotating the gyro, which undesirably slows down the surveying method.
A method for determining geographical data of a trajectory is further known from NL-C-1017128, which describes a method for measuring a borehole. The borehole extends between a first and a second location, from which DGPS (Differential Global Positioning System) coordinates are measured. A sensing unit comprising an optical gyroscope is moved from the first to the second location for measuring the borehole. The optical gyroscope performs measurements on the first location, a plurality of intermediate locations and at the second location. A drift which occurs in the calculated track is compensated by determining the difference between the measured DGPS coordinates of the second location and gyroscope measurement of the second location and proportionally correcting each of the measurements on the intermediate locations. Such a proportional correction of measurements is however only possible without adversely affecting the accuracy of the data if the trajectory or borehole is substantially straight or has a substantially continuous bend. In case of an arbitrary trajectory, such proportional correction is insufficient for obtaining data of sufficient accuracy.